As a subclass of commercially available polymers, polyurethane elastomers have several properties whose advantages confer unique benefits on them. Typically, polyurethanes and the related polyureas show high abrasion resistance with high load bearing, excellent cut and tear resistance, high hardness, and resistance to ozone degradation, yet are portable and castable. Compared to metals, polyurethanes are lighter in weight, less noisy in use, show better wear and excellent corrosion resistance while being capable of less expensive fabrication. Compared to other plastics, polyurethanes are non-brittle, much more resistant to abrasion, and exhibit good elastomeric memory. Polyurethanes find use in such diverse products as aircraft hitches, bushings, cans, gaskets, star wheels, washers, scraper blades, impellers, gears, and also serve as coatings in a wide variety of uses.
Part of the utility of polyurethanes (and polyureas) derives from their enormous diversity of properties resulting from a relatively limited number of reactants. Typically, polyurethanes are prepared on site by reacting (curing) the terminal isocyanate groups of a monomeric polyisocyanate or of a low molecular weight prepolymer with the isocyanate-reactive hydrogens of a polyfunctional compound so as to form high polymers through chain extension and, in some cases, crosslinking. Urethane prepolymers are adducts of polyisocyanates and polyhydric alcohols as exemplified by the 2:1 adducts of a diisocyanate and a diol, and urea prepolymers are adducts of monomeric polyisocyanates and polyamines. Diols, and especially alkylene diols, are the most common isocyanate-reactive materials used as curing agents and lead to linear polymers by chain extension. Where a triol or a higher polyhydric alcohol is used crosslinking occurs to afford a non-linear polymer. Although other polyfunctional materials, especially diamines, are theoretically suitable, with but a few exceptions none have achieved commercial importance as a curing agent. The major exceptions are 4,4'-methylene-di-ortho-chloroanaline, usually referred to as MOCA, and the 2,4- and 2,6-diamino-3,5-diethyl-1-methylbenzene, often referred to as diethyltoluene diamine or DETDA, curing agents which are both a chain extender and a crosslinker. More recently selected secondary diamines and polyamines have found favor as curing agents. The secondary diamines act solely as chain extenders.
Among the unsatisfied needs in the polyurethane and polyurea elastomer field is that for a product which shows excellent light stability while having the toughness of polymers made using amine-based curing agents. A disadvantage of many current products is their tendency to yellow in sunlight, whereas it would be highly advantageous to have products which show no darkening with time for particular applications such as coatings generally, elastomers such as topcoats for automobiles and outdoor implements, for roofs, coatings for bridges and decks, and for certain adhesives.